1. Field of the Invention
The present invention relates to a differential transmission circuit used to transmit a differential signal including first and second signals of phases opposite to each other as well as to an electronic apparatus equipped with the differential transmission circuit.
2. Description of the Related Art
To achieve high speed and high definition, recent digital copiers and digital cameras need to transmit a large amount of digital signals at high speed. This increases the number of devices performing simultaneous switching in driver ICs adapted to output digital signals as well as the switching frequency of the devices. For stable operation of the large-scale, fast-acting ICs, it is necessary to stabilize a power source voltage supplied to the ICs. For that, a large number of capacitors are placed around the ICs mounted on a printed circuit board, thereby stabilizing the power source voltage of the ICs.
The capacitors and ICs are interconnected via a power source line and ground line on the printed circuit board. Consequently, a high frequency current supplied to the ICs from the capacitors flows to a parasitic inductance component, thereby causing fluctuation of power source potential (power source noise). The power source noise could generate electro magnetic interference (EMI) using a cable or the like connected to the printed circuit board, as an antenna and thereby cause malfunctions of other electronic apparatus. Thus, it is necessary to suppress EMI caused by the power source noise.
On the other hand, for high-speed transmission of digital signals, differential transmission systems which are resistant to extraneous electromagnetic noise and cause less electromagnetic interference are used widely. Also, there are various standards, typical examples of which include PECL (Pseudo-Emitter Coupled Logic), LVDS (Low Voltage Differential Signaling), and PCI-express (Peripheral Component Interconnect).
The differential transmission systems vary in features depending on costs and power consumption as well as on standards including a prescribed signal amplitude and frequency band, and thus there are cases in which transmitting and receiving sides use separate transmission standards according to their respective purposes. Since different signal levels (signal amplitudes and center voltages) are prescribed by different standards, it is necessary to provide a level shift circuit, i.e., a circuit adapted to adjust the signal level of signals output by the transmitting side to the standards of the receiving side.
Conventionally, in the level shift circuit for differential transmission, a resistor group made up of three resistive elements is provided per differential signal line and two resistor groups are installed in parallel between a power source line and ground line for a total of six resistive elements (see Japanese Patent Application Laid-Open No. 2007-324799). In the level shift circuit, by adjusting the value of each resistor connected to the differential signal line, direct-current voltage levels of signals output from the transmitting side can be adjusted to the standards of the receiving side.
However, according to Japanese Patent Application Laid-Open No. 2007-324799 described above, in the level shift circuit, the power source line/ground line is connected directly to the differential signal lines via the resistive elements, and thus power source noise generated on the power source line/ground line propagates to the differential signal lines via the resistive elements in the resistor groups. Furthermore, a positive side and negative side of the differential signal line are connected to the same power source line via the resistive elements of the level shift circuit, causing power source noise to flow equally into the positive side and negative side of the differential signal line from the power source line. Consequently, the power source noises generated on the positive side and negative side of the differential signal line have the same phase. This is also true when power source noise flows in through the ground line. Therefore, there is concern that electromagnetic interference may be radiated using a cable or the like of the differential signal line as an antenna.